Phylogenetic analysis of the surface proteins of influenza A (H5N1) viruses isolated in Asian and African populations.

Highly pathogenic H5N1 virus can infect a variety of animals and continually poses a threat to animal and human health. Here, phylogenetic analysis of the hemagglutinin and neuraminidase genes indicated that the hemagglutinin gene of all human isolates, although very similar to each other, fell within different clades corresponding to antigenically distinguishable variants. Likewise, the N1 neuraminidase gene forms a clade that is evolutionarily distinct from previously characterized N1 neuraminidases. So, although all H5N1 viruses were derived from ancestors circulating in south-east Asia more than ten years ago, since 2003 they have evolved into geographically distinct groups within each country.